The present invention relates to an airfoil for applications, such as rotary wing aircraft, in which the angle of attack changes rapidly and continuously and, more particularly, to an airfoil designed to control dynamic stall.
Rotary wing aircraft have gained wide popularity, both in the military and in commercial applications, primarily because of their vertical takeoff/landing capabilities and their ability to hover. These capabilities, however, are accompanied by severe limitations such as relatively low maximum flight speeds (typically 150 knots), maneuverability limitations, as well as high maintenance costs resulting from large oscillatory loads on the rotor mechanism. These limitations have their source in a phenomenon known as dynamic stall. Dynamic stall is a highly complex phenomenon containing the traditional problems of transition, turbulence, separation, reattachment, etc., all encapsulated in an unsteady and sometimes three-dimensional and compressible environment. As a natural consequence of its complexity, dynamic stall creates a major problem for helicopter rotor designers. The root of the design problem is an incomplete understanding of the fundamental mechanisms which give rise to the flow structures which characterize dynamic stall. This lack of understanding translates directly into severe limitations in the prediction of rotor-loads, which manifests as a major rotor design constraint.
The dominant feature characterizing dynamic stall on an airfoil is a strong vortical flow, which begins near the leading-edge, enlarges, and then travels downstream along the airfoil. This so-called dynamic stall, or leading-edge, vortex (DSV) consequently brings about abrupt losses in lift as well as sharp increases in drag and strong pitching moments (L. W. Carr, "Progress in the analysis and prediction of dynamic stall", Journal of Aircraft, Vol. 25 No. 1, pp. 6-17 (1988)). The rotor designer clearly wants to avoid these undesirable features and to this end almost all research into dynamic stall suppression focuses on controlling or eliminating the leading-edge vortex. Typically, some form of airfoil geometry modification is made (e.g. leading-edge slat), or boundary-layer control (BLC) is employed (e.g. blowing or suction), where these changes are geared specifically to the leading-edge region where the vortex originates. These attempts at containing the DSV are confined to various experimental configurations and numerical studies and have not, as yet, found application in military or civil aviation.
The origin or inception of the dynamic stall vortex is a natural starting point for studies which seek to eliminate or modify it's effect. To date, without exception, all attempts to control the vortex focus on its genesis, because it is ". . . difficult to devise an effective scheme to manipulate this energetic structure after it moves away from the leading-edge . . ." (C. Shih, L. M. Lourenco and A. Krothapalli, "Investigation of flow at the leading and trailing edges of a pitching-up airfoil", AIAA Journal, Vol. 33, No. 8, pp. 1369-1376 (1995)). The following is a brief review or various dynamic stall control techniques.
K. L. McCloud III, L. P. Hall and J. A. Brady ("Full-scale wind tunnel tests of blowing boundary layer control applied to helicopter rotor", NASA IN D-335I, 1960) were the first to attempt boundary-layer control of a helicopter rotor. Performing experiments on a full-scale rotor, where blowing emanated from a leading-edge nozzle, they observed a delay in retreating-blade stall. They also suggested cyclic blowing, where BLC is applied only on the retreating blade. In a water-tunnel study, McAlister (see Carr, 1988) ascertained by means of flow visualization that the leading-edge vortex was significantly modified for a momentum coefficient C.sub.82 =6% and that the vortex was contained at C.sub..mu. =45%. G. A. Addington, S. J. Schreck and M. W. Luttges ("Static and dynamic flow field development about a porous suction surface wing", AIAA-92-2628-CP, 1992) applied upper-surface leading-edge suction in an attempt to control dynamic stall on an airfoil undergoing ramp-type motion. Dynamic stall was suppressed for pitch-rates .alpha..sup.+ &lt;0.05. In an attempt to quantify the effect of the transition region effects, Green & Gilbraith ("An investigation of dynamic stall through the application of leading edge roughness", Paper No 137, 18.sup.th European Rotorcraft Forum, Avignon, France 1992) investigated the effect of leading-edge transition strips on dynamic stall of an airfoil experiencing ramp-type pitching motion. Significant effects on the upper surface pressure distribution were observed, but quantitative improvements in aerodynamic effects have not been reported. "Air pulses" delivered from a slot at 0.2c, on the upper surface of an oscillatory pitching airfoil, were investigated by M. W. Luttges, M. C. Robinson and D. A. Kennedy ("Control of unsteady separated flow structures on airfoils", AIAA-85-0531, AIAA Shear Flow Control Conference, 1985). From flow visualization studies, an "enhanced flow adherence to the airfoil surface" was observed, when the forcing Strouhal number F.sup.+, was greater than 0.25. In addition, under these conditions the air pulses gave rise to an "ongoing process of vortex formation".
L. W. Carr and K. W. McAlister ("The effect of a leading-edge slat on the dynamic stall of an oscillating airfoil", AIAA Paper 83-2533, AIAA/AHS Aircraft Design System and Operations Meeting, 1983) were the first to investigate an airfoil with a leading-edge slat in a dynamic stall environment, and ascertained that the characteristic effect of the DSV was eliminated from the lift and pitching-moment characteristics. C. Tung, K. W. McAlister and C. M. Wang ("dynamics stall study of a multi-element airfoil", 18th European Rotorcraft Forum, Avignon, France, Sep. 15-18, 1992) carried out water tunnel experiments which confirmed the effectiveness of the leading-edge slat as a stall suppression device. It has been pointed out, however, that the addition of slats to a typical in-flight rotor configuration would seriously compromise it's structural integrity. P. Freymuth, S. Jackson and W. Bank W ("Toward dynamic separation without dynamic stall", Experiments in Fluids, Vol. 7, pp. 187-196 (1989)) performed flow visualization studies of a pitching wedge-shaped airfoil with a rotating cylindrical nose. In so-doing, they have identified a separated shear layer without the leading-edge vortex. Y. H. Yu, S. Lee, K. W. McAlister, C. Tung and C. Wang C ("Dynamic stall control for advanced rotorcraft application", AIAA Journal, Vol. 33 No. 2, pp. 289-295(1995)) examined the concept of the drooped leading-edge such that flow ". . . can pass easily around the leading-edge . . ." without generating the characteristic vortex. In a comparison with the undrooped case, lift hysteresis was reduced, and it was claimed that the characteristic increase in drag coefficient C.sub.D and large negative moment coefficient C.sub.M are reduced by about 40%. The concept of "variable leading-edge droop" was also proposed.
Nevertheless, all methods of dynamic stall control that have been attempted heretofore have been less than satisfactory. There is thus a widely recognized need for, and it would be highly advantageous to have, a more satisfactory method of dynamic stall control for airfoils, particularly in rotary aircraft applications, than the methods known in the art.